Water solution coating compositions have been employed for diverse purposes, but it has been difficult to obtain the good resistance to extraction by hot water and good odor and flavor characteristics which are important to enable application of the coatings to sanitary cans.
In our applications Ser. Nos. 885,036 and 18,887, a relatively low molecular weight polyepoxide, such as a diglycidyl ether of a bisphenol, is esterified with an at least approximately stoichiometric proportion of monocarboxylic acid selected from benzoic acid, a C.sub.1 -C.sub.8 alkyl-substituted benzoic acid, or a C.sub.6 -C.sub.10 alkanoic acid to produce an ester derivative substantially free of epoxy functionality. The resulting hydroxy functional epoxy ester is then polyesterified with a small amount of monoethylenic dicarboxylic acid which resists homopolymerization, such as fumaric acid, and this unsaturated polyester is then copolymerized with monoethylenic monomers including carboxyl-functional monomer to provide a copolymer which is dispersed in water with the aid of an amine.
It is also known to copolymerize monoethylenic monomers including carboxyl-functional monomer in the presence of a polyepoxide, but this leaves most of the epoxide groups unreacted and much of the copolymer formed remains unassociated with the polyepoxide.
These systems are each inadequate in certain respects, particularly because organic amines are needed for good solubility and stability in water. It is desired to use ammonia because organic amines introduce the possibility of nitrosamine formation, and also because ammonia is more economical and its vapors are less troublesome.